The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell power plant that utilizes an antifreeze solution passing through components of the power plant.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate or support layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate or support layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cM.E.A.xe2x80x9d, and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell as a liquid through a porous water transport plate, or by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions.
Known fuel cells typically utilize an open or closed thermal management or coolant system supplying a flow of cooling fluid through a porous or sealed cooler plate within the fuel cell to maintain the cell within an optimal temperature range. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize a conventional antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in such a closed coolant system having a sealed cooler plate. However, such antifreeze solutions are not acceptable in an open coolant system because they are known to be adsorbed by and poison the catalysts that form electrodes. Furthermore, those antifreeze solutions have low surface tensions which result in the solutions wetting any porous, wetproofed support layers adjacent cell catalysts, thereby impeding diffusion of reactant fluids through the support layers to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of such conventional antifreezes is high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams.
A fuel cell power plant includes a fuel cell or fuel cell stack to generate electricity and a variety of systems to support the fuel cell stack. For example, if the plant is to be utilized to power a transportation vehicle, it is necessary that the power plant be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset losses from reactant fluids exiting the plant in order to efficiently operate the plant. Any water exiting the plant through a plant process exhaust stream consisting of a cathode exhaust stream of gaseous oxidant and/or an anode exhaust stream of fluid exiting the anode side of the fuel cell or a burner exhaust stream must be balanced by water produced electrochemically at the cathode electrode and water retained within the plant. To maintain water self-sufficiency, it is common that the plant include a water recovery device, controls, and piping to recover and direct water into the fuel cell stack to maintain proper wetting of the PEM electrolytes, and humidity of the reactant streams, etc.
Additionally, it is known that some fuel cell power plants operate on pure hydrogen gas, while others utilize a reformate wherein a hydrogen enriched reducing fluid is formed from any of a variety of hydrocarbon fuels by fuel processing components including for example use of steam mixed with the fuel at high temperatures within a reformer, as is well known in the art. If a fuel cell power plant included a steam generator for such fuel processing components, as with the water recovery device, the water in such components would have to be protected against freezing, such as by use of an antifreeze solution.
Water retention by the recovery devices of such fuel cell power plants will vary depending upon power output of the plant. For example, the water recovery device may be a condenser positioned to pass a process exhaust stream exiting the fuel cell in heat exchange relationship with a cooling fluid to condense and recover water within the process exhaust stream. When the power output of the plant is low, more water will be recovered by the condenser than when the power output is high. If the water recover ed by the condenser is mixed with an antifreeze to prevent freezing and then directed to power plant components, an excess recovery rate of the water may so dilute the antifreeze within the water management system and other power plant components that the water may freeze. Another problem with excess water recovery is that the dilute d antifreeze may overflow from the system resulting in a loss of antifreeze.
Accordingly, there is a need to control a concentration of antifreeze within the fuel cell power plant so that an antifreeze solution passing through the plant is maintained within a proper antifreeze concentration range.
A direct antifreeze solution concentration control system for a fuel cell power plant is disclosed for controlling a concentration of a direct antifreeze within a direct antifreeze solution cooling fluid passing through the plant. The control system includes at least one fuel cell for generating electricity from reducing fluid and process oxidant reactant streams having an electrolyte secured between an anode and cathode catalyst; a thermal management system that controls a temperature within the fuel cell including a porous water transport plate secured in direct fluid communication with the cathode catalyst that receives water adjacent the cathode catalyst during generation of electricity, wherein the direct antifreeze solution passes through the water transport plate; a water recovery device in fluid communication with a process exhaust stream exiting the fuel cell within a plant exhaust line and in fluid communication with a cooling fluid for recovering water from the process exhaust stream and for distributing the recovered water within the power plant; a direct antifreeze reservoir in fluid communication with the thermal management system for selectively supplying the direct antifreeze to the direct antifreeze solution passing through the porous water transport plate; and, a process exhaust by-pass line in fluid communication between the fuel cell and a plant exhaust vent that selectively directs some or all of the process exhaust stream to by-pass the water recovery device and to pass out of the plant through the plant exhaust vent.
In alternative embodiments of the direct antifreeze concentration control system, the water recovery device may be a direct mass and heat transfer device that directs the process exhaust stream exiting the fuel cell to pass within the device in mass transfer relationship with the process oxidant stream entering the plant so that water vapor and heat within the process exhaust stream pass directly through a mass transfer medium means of the device into the oxidant stream entering the fuel cell. In such an embodiment during a period of excess water recovery, the process exhaust by-pass line may selectively direct some or all of the process exhaust to avoid the direct mass and heat transfer device and pass out of the plant through the plant exhaust vent, thereby reducing an amount of water retained within the plant. Additionally or alternatively, the concentration control system may include an oxidant mass-transfer device by-pass line that selectively directs some or all of the process oxidant stream to by-pass the mass and heat transfer device prior to entering the fuel cell to also reduce the amount of water recovered and retained within the power plant through the process oxidant stream entering the plant.
In another embodiment, the water recovery device may be a condenser water recovery device that directs the process exhaust stream to pass in heat exchange relationship with a condenser cooling fluid, wherein the condenser cooling fluid also passes through a heat rejection apparatus and flow control valve for controlling a temperature of the condenser cooling fluid within the condenser. In a further embodiment, the concentration control system may include a boiler in fluid communication with the thermal management system that boils the direct antifreeze solution to produce steam in fluid communication with a steam separator and steam exhaust valve and vent for selectively directing steam out of the power plant through the steam exhaust vent.
Because the porous water transport plate of the thermal management system is in direct fluid communication with the cathode catalyst of the fuel cell so that there is no barrier to liquid or vapor flow between the water transport plate and the cathode catalyst, it is necessary to utilize a direct antifreeze solution because ordinary ethylene glycol and water or propylene glycol and water types of antifreeze solutions would move from the water transport plate to contact and poison the cathode catalyst. The direct antifreeze solution is an organic antifreeze solution that is not volatile at cell operating temperatures, such as glycerol. For purposes herein, xe2x80x9cnon-volatilexe2x80x9d is defined to mean that the antifreeze solution sustains a loss of less than 10% of its antifreeze for every 500 operating hours of the fuel cell at fuel cell operating temperatures.
In operation of the direct antifreeze solution concentration control system for a fuel cell power plant, water passing through the electrolyte from the anode catalyst, water generated at the cathode catalyst, and water within the process oxidant stream passing by the cathode catalyst may move directly into the porous water transport plate, and hence into the thermal management system. Water vapor may also move from the cathode catalyst to the water transport plate or into the oxidant stream passing by the cathode, to be recovered within the water recovery device. Water recovered by the water recovery device also moves into the thermal management system directly through the mass and heat transfer water recovery device into process oxidant stream entering the fuel cell and then into the water transport plate, or indirectly through water moving from the condenser into the boiler and then into a steam injection line for selectively injecting steam into the reactant streams entering the fuel cell to humidify the reactant streams. The thermal management system and water recovery device are therefore in direct or indirect fluid communication. Consequently, an increase in water recovered by the water recovery device would dilute a concentration of direct antifreeze within the direct antifreeze solution in the thermal management system, and could also lead to diluted antifreeze overflowing from the plant leading to loss of the antifreeze. The increased water and diluted antifreeze could also result in an ice build-up in the power plant, especially during a shut down period in a sub-freezing environment, thereby requiring a pre-start up melting system and deleterious delay in usage of the fuel cell power plant. Such an ice build up could also result in mechanical damage to various components of the plant.
When a control sensor, such as a viscosity sensor, senses a dilution of the direct antifreeze within the direct antifreeze solution within the power plant, the concentration control system may control the process exhaust by-pass line to direct some or all of the process exhaust stream to by-pass the water recovery device. Additionally, if the water recovery device is a direct mass and heat transfer device, the concentration control system may control the oxidant mass transfer by-pass line to direct some or all of the process oxidant stream entering the fuel cell to by-pass the mass and heat transfer device, to thereby decrease the water vapor in the process oxidant stream, resulting in less water entering the thermal management system through the porous water transport plate, which results in an increase in the relative proportion of direct antifreeze in the direct antifreeze solution within the thermal management system.
If the water recovery device is a condenser, then the concentration control system may control the heat rejection apparatus or condenser cooling fluid flow control valve to increase the temperature and/or decrease the flow rate of the condenser cooling fluid passing through the condenser to thereby decrease a rate of condensation of water in the process exhaust stream passing through the condenser. That causes more water to be removed from the plant within the process exhaust stream passing through the plant exhaust vent. Similarly, if the plant includes a boiler, steam separator and steam exhaust valve and vent, the concentration control system may control the steam vent to direct steam to be vented out of the power plant. The steam separator may also be controlled to send some of the direct antifreeze separated from the steam back to the thermal management system to increase the concentration of direct antifreeze in the direct antifreeze solution within the thermal management system. If the sensor senses the concentration of direct antifreeze descending below a specific level, the concentration control system also controls the direct antifreeze reservoir to send some of the direct antifreeze from the reservoir into the thermal management system, to thereby increase the concentration of direct antifreeze within the direct antifreeze cooling fluid passing through the fuel cell and other power plant components.
Accordingly it is a general object of the present invention to provide a direct antifreeze solution concentration control system for a fuel cell power plant that overcomes deficiencies of the prior art.
It is a more specific object to provide a direct antifreeze solution concentration control system for a fuel cell power plant that minimizes dilution of direct antifreeze within a direct antifreeze solution cooing fluid within the plant.
It is yet another object to provide a direct antifreeze solution concentration control system for a fuel cell power plant that increases a concentration of direct antifreeze within the direct antifreeze solution cooling fluid whenever the concentration of direct antifreeze decreases below a minimum desired concentration.
It is another object to provide a direct antifreeze solution concentration control system for a fuel cell power plant that controls a concentration of direct antifreeze within a direct antifreeze solution cooling fluid within all components of the power plant.
It is a further specific object to provide a direct antifreeze solution concentration control system for a fuel cell power plant that controls a concentration of direct antifreeze within a direct antifreeze solution cooling fluid in direct fluid communication with an anode or cathode catalyst of the fuel cell without poisoning the catalysts.